Recently, with the increasing focus on the environmental protection, water-dispersible polyisocyanates have been more and more widely used in each field because they are environmental friendly. First of all, they can be used as cross-linking agents to prepare two-component aqueous coatings with good properties; Secondly, they can be used in aqueous dispersion adhesives as additives; besides, they can be used to cross-link other aqueous dispersions in textile finishing and printing, and can be used as auxiliaries for paper humidifying treatment.
At present, water-dispersible modified polyisocyanates can be generally divided into two categories: non-ion modified polyisocyanates and ion modified polyisocyanates. The non-ion modified polyisocyanates mainly employ polyethers as raw materials for modificatoin. Although polyisocyanates that modified with polyethers have been widely accepted in the market, they still have many defects: for example, a large amount of polyethers must be used to provide polyisocyanates a good water-dispersity, which significantly decreases the concentration of isocyanates in the polyisocyanate system; secondly, the assistance of large shearing force is required for modified polyisocyanates to be dispersed completely in water; moreover, a huge amount of polyethers will be present in the solidified coating films, which will influence the water resistance of the coating films permanently.
In order to solve these problems, hydrophilic modifications of polyisocyanates have been achieved by adding hydrophilic ionic groups into polyisocyanates, and successes have been widely achieved.
In EP0443138A and EP0548669A, a method of introducing carboxy groups to modify polyisocyanates is provided. When carboxy groups are neutralized, modified polyisocyanates can be dispersed in water easily without a high shearing force. But the defects are polyisocyanates modified with carboxylic acids cannot be dispersed in the system with pH less than 5; besides, ions formed after neutralization by tertiary amines are good catalysts for self-polymerizations of isocyanates, which will result in a decrease of storage stability of modified poyisocyanates. Therefore, polyisocyanates modified with carboxy groups can only be stored in unneutralized forms, and will be neutralized before being used, which is hard and tedious to operate.
In CN101754990A, a method of using 4-aminotoluene-2-sulfonic acid to modify polyisocyanates is provided, said modified polyisocyanates can be dissolved in water very easily after neutralization. However, it is required in said method to use a certain amount of polyethers, which results in the decrease of the water-resistance of coating films; besides, the sulfonic acids used comprise benzene ring, which results in the decrease of the property of anti-yellowing of coatings.
In DE4433929A, a preparation of polyisocyanates modified with sulfonic acid using small molecular sulfonic acid (2-hydroxy propane sulfonic acid) and big molecular sulfonic acid (hydroxyl-terminated polyether sulfonic acid, such as the Tegomer series products) is provided, after ionization, said modified polyisocyanates can be well dispersed in water. However, 2-hydroxy propane sulfonic acid used herein is carcinogenic, and special big molecular sulfonates are generally provided in the form of sodium salts in industry, because a sulfonic acid neutralized by a strong base is poorly compatible with the other component in a two-component coating, resulting in an opaque two-component system and inhomogeneous coating. And sodium ion will be left in the coating films and influence the water-resistance of coating films permanently.
In CN1190450C, modified polyisocyanates are prepared using 3-(cyclohexylamino)-propanesulfonic acid and 2-(cyclohexylamino)-ethanesulfonic acid, the obtained modified polyisocyanates can be dispersed in water homogeneously without a high shearing force, and polyisocyanates modified with sulfonates neutralized by tertiary amines possess very good storage stability. However, the reaction time of the preparation of polyisocyanates modified with sulphamic acids of said patent method is long, the types of sulphamic acids that can be used to carry out hydrophilic modifications of polyisocyanates by such a reaction are very limited, only 3-(cyclohexylamino)-propanesulfonic acid and 2-(cyclohexylamino)-ethanesulfonic acid are disclosed, and it is mentioned explicitly that many other sulphamic acids with very similar structures cannot react with polyisocyanates even under more drastic conditions.